There is an increasing demand for reduction in size, weight, and thickness of portable information equipment and portable audio equipment such as a portable telephone and a mobile computer so as to improve their portability. It would not be an exaggeration to state that a battery power source is the key to realize such a reduction. In particular, there is a strong demand for reduction in weight and thickness of a battery power source device using a rechargeable battery.
As a rechargeable battery constituting a battery power source device of portable equipment, a nickel-cadmium battery, a nickel metal hydride battery, and a lithium ion rechargeable battery are used. Above all, the lithium ion rechargeable battery is optimal as a power source battery of portable electronic equipment because it has high mass energy density and weight energy density. However, in addition to its extremely high energy density, the lithium ion rechargeable battery uses a flammable organic solvent as an electrolyte solution. Therefore, it is necessary to ensure safety in the case where the battery gets into an abnormal state. It is believed that the abnormal state is caused by an electrical, mechanical, thermal factor, and the like. The abnormal state refers to, for example, overcharge, short circuit, pressing, overheating and the like. The lithium ion rechargeable battery is provided with a protective function such as a PTC (Positive Thermal Coefficient) element for protecting the battery against short circuit or overheating or a battery protecting circuit for protecting the battery against overcharge, overdischarge and the like, or a safety function such as a gas exhaust vent for externally exhausting an abnormal internal pressure so as to prevent the battery from exploding. The safety function is provided for the battery itself, whereas the battery protecting circuit or the PTC constituting the protective function is housed with the battery in a pack case so as to be configured in a battery pack form.
FIG. 8 is an exploded view illustrating an exemplary structure of a battery pack using a flat prismatic lithium ion rechargeable battery. A lithium ion rechargeable battery 60 formed in a flat prismatic shape, and a circuit board 61 and a PTC 62, which form a battery protecting circuit, are housed within a pack case composed of an upper case 51 and a lower case 52. Charge and discharge can be achieved through external connection terminals 54 provided on a side face of the lower case 52.
The reduction in thickness of portable electronic equipment is considerably pursued so as to improve the portability. Therefore, there is an increasing demand for reduction in thickness of a battery pack serving as a power source. In order to realize the reduction in thickness of the battery, however, a battery capacity should be sacrificed. In addition, there is a limit to reduce the thickness in view of the structure of the battery.
Since there is a limit to reduce the thickness of the battery itself, a thickness of a pack case is necessarily reduced so as to reduce the thickness as a battery pack. The case, which is a resin molding product, is required to have about 0.25 mm as the minimum thickness allowing the molding. The quality of a molding cannot be kept due to lowered moldability or a degraded surface property in resin molding at the minimum thickness, thereby lowering the mass-productivity. Moreover, the thickness of the case is reduced and the like to lower the strength of the case. Therefore, there is a limit of the reduction in thickness so as to obtain the strength that is high enough to protect the battery and the other components.
The present invention has an object of providing a battery pack that is reduced in thickness by using a film as a surface of a pack case, which surface otherwise constitutes a limit thickness of molding in resin molding, so as to reduce a thickness of the pack case to the limit.